Time delay snap switch with bimetallic pull ribbon



Dec. 13, 1966 G. G. MERKL 3,291,936

TIME DELAY SNAP SWITCH WITH BIMETALLIC PULL RIBBON Original Filed Sept. 24, 1963 2 Sheets-Sheet 1 INVENTOR. g grs Mae/(4 G- G. MERKL Dec. 13, 1966 TIME DELAY SNAP SWITCH WITH BIMETALLIC PULL RIBBON 2 Sheets-Sheet '2' Original Filed Sept. 24, 1963 I I l I l I INVENTOR. fanny? 0759K;

BY aw LZALLM ATTORNEYS United States Patent Ofilice 3,291,936 TIME DELAY SNAP swrrcn wrrn BrMETALLrc PULL RIBBON George G. Merkl, New Milford, NJ, assignor, by mesne This is a division of application, Serial No. 311,133, filed September 24, 1963.

The present invention relates to a novel snap action switch that is particularly adapted for use in warning signal receivers.

The snap switch of the invention is of the general type disclosed in United States Patent No. 3,037,102 but differs therefrom in that the actuating means of the switch is a bi-metal member instead of a flexible pull wire. The bimetal member is a ribbon, made of two metals with different coefiicients of expansion, which is fixed under tension between the finger extending outwardly from the frame of the switch and the lever arm anchored on the snap plate of the switch with the metal with the lower coefiicient of expansion facing the snap plate. With this arrangement, when the ribbon is heated it at first curls in a manner which increases the force that it exerts on the snap plate to keep the snap plate against the normally closed contact and thereafter as the temperature increases it increases in length and straightens to decrease the force it exerts on the snap plate and thereby allows the plate to snap against the normally open contact. In this way the bi-metal pull ribbon supplies a time delay before the initiation of the snap cycle that is useful in certain applications and also insures good contact pressure.

One application for such a snap switch is in a warning signal receiver. This warning signal receiver is designed to be plugged into conventional 115 volt 60 cycle power lines to detect signals when they are super-imposed upon the power lines to give a warning, for example, of fires and air raids. The delay provided by the present snap switch when the snap switch is incorporated into this receiver prevents spurious signals and noise from triggering the alarm.

For a better understanding of the present invention, and the switch embodying the same, reference should be had to the accompanying drawings of which:

FIGURES 1 and 2 are respectively front and side views of the switch with the bi-rnetal pull ribbon;

FIGURE 3 is a graph showing the relative changes in length with changes in temperature of the two metals employed in the pull ribbon; and

FIGURE 4 is a schematic diagram of a receiver employing the switch with a bi-metal pull ribbon.

Referring to FEGURES 1 and 2, a rectangular plate 2 is provided with two slots 4-4 which extend for the major length of the plate. The slots 44 define a central vane 6 and two side members 8. The central vane 6 varies in width from a minimum adjacent the central section to a maximum at the closed ends of the slots and each side member 8 of the plate is provided with a crimp 10 which shortens its length relative to that of the central vane 6.

One end 12 of the plate is formed with a central raised section 14. This raised portion 14 provides the bias at the free end of the plate and insures against reversal of curvature of the central vane during operation of the device. The raised portion or bias 14, in conjunction with the crimps 10, causes the adjacent end of the central vane to curve outwardly from the body of the plate. The metal of the plate adjacent the other end of the central vane 6 is also forced outwardly away from the plane of 3,291,936 Patented Dec. 13, 1966 the plate as indicated at 16 to provide a bias at that end of the plate which likewise insures that the strip adjacent that end will bow outwardly from the plane of the plate. Preferably, as shown best in FIG. 1, the width of the central vane 6 is slightly greater at the end adjacent the bias 16.

The end of the plate adjacent raised portion 16 is welded or otherwise fixed to a frame element 18 of the switch, and the other, or free end of the plate, carries two contact members 20 and 21 on opposite sides thereof. An L- shaped anchor member 22 of round stock has one leg welded to the convex side of the vane 6, adjacent the fixed end of the plate, with its other leg upstanding. The frame 18 has a finger 26 bent out therefrom with the free end of the finger in general alignment with the upstanding end 24 of the anchor member. A bi-metal expansible pull ribbon 28 is secured under tension to the finger 26 and t0 the upstanding end 24 of the anchor member. The pull ribbon is provided with an insulated heating wire 30 that lies in a number of side by side lengths alongside the pull ribbon. The pull ribbon and the side by side lengths of the heating wire are positioned in an insulated casing 32 to obtain maximum heat transference from the heating wire to the pull ribbon.

The switch is supported on a base 36 of insulating material, such as phenol formaldehyde resin, in which are anchored three prong terminals 38, 40 and 42 by means of lugs or tongues which extend through the base 36. The frame 18 has an offset lower portion 44 which rests on the base 36 over the terminal 38 and is clamped to the base by the lugs 46 of that terminal. The frame is provided with a generally square opening 48 within which is a support 50 carrying a fixed contact 52. Support 50 includes a lower generally square plate element 50a which is clamped to the lower part of frame 18 by inturned flanges 54 of the frame, and spacers 56 of mica or the like on each side of the plate element electrically insulates the element from the frame. The contact 52 is welded to one face on the support 5t within the opening 48 of the frame. The support 50 also has a rearwardly extending tab 5% on the support 52 to provide means for adjusting the position of the fixed contact, and for the electrical connection of the contact 52 to the terminal 40, the support 50 is provided with an outwardly extending finger 500.

Also clamped between the flanges 54 and electrically insulated therefrom and from the plate element 50a, by mica spacers 56, is a plate element 58a forming a lower extension of a support arm 58 for a contact 60. The contact 6t) is aligned with contact 52 and has an outwardly extending tab 58b thereon for adjustment of the spacing of the stop from the snap plate 2. For electrical connec nection of the contact 60 to one end of the heater wire 30, the support 58 is provided with an outwardly extending finger 580. The other end of the heating wire is attached to prong 42.

The upper end of frame 18 is formed along its sides with portions 62 offset from the plane of the frame to provide spaced areas on which the snap plate 2 is welded. The free end of the snap plate extends between the contacts 52 and 66 with one plate carried contact 20 aligned with the contact 52 and the other plate carried contact aligned with contact 6%. A suitable cover (not shown) is crimped to the periphery of the base 36 to complete the assembly.

After mounting the snap plate 2 on the frame element 18 and attaching the pull ribbon 28 taut between the anchoring finger 26 and the upstanding leg 24 of the L-shaped anchor member, the switch is adjusted by bending -or moving the finger 26 to increase the tension in the pull wire. The introduction of tension into the pull wire, through movement of the L-shaped anchor member on the vane, provides a longitudinal pull on the central vane 6 which causes the intermediate central section of the vane to go into concavity while the end sections of the vane remain in convexity. In this position pressure begins to build up between contact 20 and contact 52. When the tension is further increased by movement of the finger 26, suflicient force is provided to cause the central portion of the vane 6 to pass beyond the plane of the plate and assume the contour of a relatively flat M. At a given tension in the pull wire the central part of the plate is forced through a position of unstable equilibrium at which instant some of the force accumulated in the vane by the longitudinal pull of the ribbon is released and operates to snap the free end of the plate away from contact 52 and against contact 60.

The two metals in the pull ribbon are copper, or an alloy, consisting of 46% nickel and the balance iron and Nilvar, an alloy consisting of 36% nickel and the balance iron. The two metals are positioned one underneath the other with the Nilvar underneath adjacent the surfaces of the arm and the finger. In FIGURE 3 the rates of expansion of these two materials with temperature are shown. As can be seen at first the rate of expansion of the copper is much greater than the rate of expansion of the Nilvar. Therefore, the bi-metal ribbon will curl in the direction of the Nilvar side of the ribbon. Since the bi-metal ribbon is fixed at one end to the finger 26, this means that the other end of the ribbon will move downwardly and exert a downward force on the arm 24 thereby increasing the force which the ribbon exerts on the plate 2 to keep the end of the plate 2 against contact 60. However, when the temperature of the bi-metal ribbon reaches t (370 F.) there is a sharp increase in the rate of expansion of the Nilvar. At this temperature the length of the pull ribbon starts increasing appreciably and rapidly thereby rapidly decreasing the force the ribbon exerts on the plate 2 to hold the end of the plate 2 against the contact 60 so that the free end of the plate snaps from its position against contact 68 back to its position against contact 52. When this occurs, the circuit to the coil is opened by the separation of contacts 21 and 68 and the pull ribbon therefore starts cooling. In the illustrated embodiment, movement of the end of the snap plate away from contact 60 occurs between t (370 F.) and t (570 F.). Cooling of the pull wire continues until the ribbon contracts sufliciently to make the free end of the plate snap back against contact 60, whereupon the circuit to the heater 30 is closed again. In the illustrated embodiment it takes about seconds to heat the pull ribbon sufliciently to snap the end of the plate against contact 52 and 40 seconds to cool the pull ribbon sufliciently to snap the plate against contact 60.

' Therefore the use of the bi-metal pull ribbon instead of mono metal pull ribbon increases the length of the flashing cycle significantly. At the same time the operating characteristics of the switch are not deleteriously affected since the rapid change in length of the pull ribbon insures good contact pressures and a rapid change in position from one contact to another.

An example of a device where the snap switch of the invention is advantageously employed is the receiver shown in FIG. 4. In this receiver, the coil 64 of an electro magnet 66 is attached in series with a capacitor 68 between the two prongs 70 and 72 of a two terminal wall plug. This electro magnet 66 has two straight flux guiding arms 74 and 76 whose ends are arranged in closely spaced relationship to a spring steel strip 78 attached to the casing 80, so that the strip will be attracted to the arms 74 and 76 when the electromagnet is energized. The coil also has two L-shaped flux guiding arms 82 and 84 having closely spaced ends 86 and 88. Positioned intermediate the ends 86 and 88 is an iron reed 90 on which is mounted a contact 92.

In a spaced relationship to this first contact 92 is a second fixed contact 94. The fixed contact 94 is connected to the prong 70 of the wall plug and the reed 90 is connected to one end of the heater coil 30 for a snap switch 96 of the present invention. The frame 18 of the snap switch is connected to prong 72 of the plug and the normally open contact 52 of the snap switch is connected to one end of a resistor 98. The other end of the resistor 98 is connected to a point 180 which is common to the capacitor 68 and the coil 64.

Connected directly between the two prongs 70 and 72 is a light 102. This light is positioned behind a glass lens positioned in an opening 103 in the casing so that when the light is lit it will be visible from the outside of the casing.

When the plug for the unit is inserted into a wall socket current will flow through lamp 102 to light it. Current will also flow through the coil 64 of the electro magnet 66 and the capacitor 68. The impedance of the capacitor is such that current flow through the coil due to the cycle 115 V. AC. line voltage is insufficient to have a significant effect on either the spring steel strip or the reed 90.

At higher frequencies the impedance of the capacitor will of course be reduced. Therefore, if for example, an 8 volt 255 cycle signal were impressed upon the normal 60 cycle 115 V. AC. line voltage, the impedance of the capacitor will permit suflicient current to flow through the coil 64 to vibrate the reed 90. In vibrating, the reed causes contacts 92 and 94 to intermittently open and close. This allows current to flow between prongs and 72, through the contacts 92 and 94, the heating coil 38, the normally closed contacts 21 and 60, the snap plate 2 and the frame 18, heating the bi-metal pull ribbon and causing it to expand. When expansion of the pull ribbon 28 is sufliciently large, the end of the snap plate will snap from the normally closed contact 60 and move against the normally open contact 52. This puts resistor 98 in parallel with capacitor 68 since current will flow between the prong 72 and coil 64 from the prong side of the capacitor 68 through the frame 18, the snap plate 2, the normally open contact 52 and the resistor 98.

The resistance of resistor 98 is quite small and therefore when it is in parallel with capacitor 68 it increases the current flow through the coil 64 of the electromagnet quite significantly. This increase in current flow sets up a strong magnetic field which is suflicient to attract the steel strip 78 towards the ends of the arms 74 and 76 against the spring force in the strip normally holding the armature away from the arms. Since the current flowing through the coil is alternating, this magnetic field will also alternate allowing the armature to be moved away from the ends of the pole pieces 74 and 76 by the above mentioned spring force when the force due to the magnetic field drops to zero. This will occur 120 times a second causing the strip 78 to vibrate back and forth and produce a buzzing sound.

While the strip 78 is producing the buzzing sound, the end of the snap plate 2 is positioned away from the contact 66 so that the bi-metal pull ribbon 28 is cooling. When it cools sufiiciently the pull ribbon 28 will compress the buckling member and cause it to snap back against contact 68 thereby taking resistor 98 out of the excitation circuit. This reduces the current flow through the coil 64 sutficiently to end the attraction of strip 78 towards arms 74 and 76, and therefore the buzzing will stop. Also, with the snap plate back against the contact 68, the circuit between heater coil and contacts 92 and 94 is closed permitting current to flow through the heater coil, starting another switching cycle for the flasher 96.

It should be obvious that as long as the 8 volt 255 cycle signal is impressed on the volt 60 cycle line voltage thesnap plate 2 will continue switching its position between contacts 52 and 60 in this manner, resulting in a series of alternate periods of buzzing and silence. However, once the 255 cycle signal has stopped, with the return of the snap plate to its position against contact 60, the amount of current flowing through the coil 64 is insuflicient to bring contacts 92 and 94 together to transmit current to the heater coil. With no current flowing through the heater coil, the snap switch stops, bringing an end to the buzzing until another 8 volt 255 cycle signal is transmitted over the power transmission lines.

The snap switch of the present invention is preferred to other snap switches because it provides the necessary lag in operation to prevent spurious signals from triggering an alarm by the receiver. Also, the length of the switching cycle and the differential between the heating and cooling portions of the cycle have been found to be particularly well suited for use in audible signaling.

The improved flasher of the present invention and one type of receiver employing it have now been described. Obviously various features of the flasher can be changed without departing from the spirit or scope of the accompanying claims.

What is claimed is:

1. In a snap switch, the combination comprising:

(a) a frame;

(b) two spaced apart stops attached to said frame;

(c) a resilient metallic plate having one end fixedly secured to the frame and the other end freely movable between the spaced apart stops, said plate having two substantially longitudinal slots therein which define a bowed centrally located vane;

(d) an expansible pull ribbon having on one side thereof a first metal with a smaller coeflicient of expansion at lower temperatures and with a higher coeflicient of expansion at higher temperatures and having on the other side thereof a second metal with a coefficient of expansion at lower temperatures which is greater than the coeflicient of expansion of the first metal at lower temperatures, said pull ribbon being attached at one end to the frame and at the other end to the plate to hold the centrally located vane in compression so that at lower temperatures the plate is positioned adjacent the first of the spaced apart stops and when some higher temperature is exceeded is positioned adjacent the second of the spaced apart stops; and

(e) an insulated electrical heating wire positioned adjacent the pull ribbon to heat the pull ribbon.

2. In a snap switch, the combination comprising:

(a) a frame;

(b) two spaced apart stops attached to said frame;

(c) a resilient metallic plate having one end fixedly secured to the frame and the other end freely movable between the spaced apart stops, said plate having two substantially longitudinal slots which define a bowed centrally located vane;

(d) anchor means on said metallic plate, said anchor means having an arm upstanding from the plane of the metallic plate that can be pivoted back and forth on said metallic plate to cause the bowed centrally located vane to curve snapping the free end of the plate back and forth between the two spaced apart stops;

(e) an expansible pull ribbon having on one side thereof a first metal with a small coefficient of expansion at lower temperatures and with a higher coefiicient of expansion at higher temperatures and having on the other side thereof a second metal with a coetficient of expansion at lower temperatures which is greater than the coeflicient of expansion of the first metal at lower temperatures, said pull ribbon being attached at one end to the frame and at the other end to the arm upstanding from the plane of the plate to position the arm so that it holds the free end of the plate adjacent one of the spaced apart stops at lower temperatures and against the other of the spaced apart stops when some higher temperature is exceeded; and

(f) an insulated electrical heating wire positioned adjacent the pull ribbon.

3. The structure of claim 2 wherein said heating wire lies alongside the pull ribbon in a plurality of side by side lengths.

4. The structure of claim 3 including a tubular insulating casing positioned around the pull ribbon and the side by side lengths of the said heating wire.

References Cited by the Examiner UNITED STATES PATENTS 2,272,021 2/1942 Riche 20013 8 3,03 7,102 5/ 1962 Schmidinger 2001 13 X 3,213,239 10/1965 Bodge 200-113 BERNARD A. GILHEANY, Primary Examiner. THOMAS MACBLAIN, Assistant Examiner. 

2. IN A SNAL SWITCH, THE COMBINATION COMPRISING: (A) A FRAME: (B) TWO SPACED APART STOPS ATTACHED TO SAID FRAME; (C) A RESILIENT METALLIC PLATE HAVING ONE END FIXEDLY SECURED TO THE FRAME AND THE OTHER END FREELY MOVABLE BETWEEN THE SPACED APART STOPS, SAID PLATE HAVING TWO SUBSTANTIALLY LONGITUDINAL SLOTS WHICH DEFINE A BOWED CENTRALLY LOCATED VANE; (D) ANCHOR MEANS ON SAID METALLIC PLATE, SAID ANCHOR MEANS HAVING AN ARM UPSTANDING FROM THE PLANE OF THE METALLIC PLATE THAT CAN BE PIVOTED BACK AND FORTH ON SAID METALLIC PLATE TO CAUSE THE BOWED CENTRALLY LOCATED VANE TO CURVE SNAPPING THE FREE END OF THE PLATE BACK AND FORTH BETWEEN THE TWO SPACED APART STOPS; (E) AN EXPANSIBLE PULL RIBBON HAVING ON ONE SIDE THEREOF A FIRST METAL WITH A SMALL COEFFICIENT OF EXPANSION AT LOWER TEMPERATURES AND WITH A HIGHER COEFFICIENT OF 